Effect of Ga substitution with Al in ZSM-5 zeolite in methanethiol-to-hydrocarbon conversion

The catalytic properties of conventional H-[Al]-ZSM-5 and gallium-substituted H-[Ga]-ZSM-5 were evaluated in the conversion of methanethiol to ethylene (CH3SH → 1/2C2H4 + H2S). Dimethyl sulfide (DMS), aromatics, and CH4 were formed as byproducts on the H-[Al]-ZSM-5 catalyst. The introduction of Ga into the ZSM-5 structure provided a high ethylene yield with relatively high selectivity for olefins. Based on the temperature-programmed desorption of NH3 and pyridine adsorption on zeolites, the strength of acid sites was decreased by introducing Ga into the ZSM-5 structure. Undesirable reactions seemed less likely to occur at weakly acidic sites. The suppression of the formation of dimethyl sulfide (CH3SH → 1/2C2H6S + 1/2H2S) and the sequential reaction of ethylene to produce aromatics provided a high yield of ethylene over H-[Ga]-ZSM-5.


Introduction
Mercaptans (thiols) are highly reactive sulfur-containing species and have unpleasant odors. The most volatile thiol is methanethiol (CH 3 SH), a sulfur species known to be released into the atmosphere. 1,2 A colorless gas, CH 3 SH, is found in natural gas and petroleum. In the rening process, thiols are oxidatively removed by a cobalt(II) phthalocyanine (discovered by William Gleim and Peter Urban of UOP) catalyst. 3 The UOP Merox™ (derived from MERcaptan OXidation) process is commercialized in over 1700 worldwide process units. 4 Other methods of removing thiols include alkali treatment, reaction with olens, and desulfurization, requiring oxygen, hydrogen, and olens. [5][6][7] The challenge in the removal process is to develop a novel method to convert CH 3 SH without the addition of reactants such as oxygen, hydrogen, and olens and with minimal byproducts and waste generation.
A few catalytic conversions of CH 3 SH to useful materials have been reported. Butter et al. used an H-ZSM-5 catalyst to convert CH 3 SH to CH 4 and DMS at 288°C. 8 Mashkina et al. reported that acid and base catalysts with various oxides converted CH 3 SH to CH 4 and DMS from 200 to 400°C. 9 Huguet recently performed a CH 3 SH conversion at low concentrations using protonated zeolites (H-ZSM-5, H-Y, and H-ferrierite) as catalysts. 10 Above 427°C, light alkanes (C1-C3), benzene, toluene, and xylene are formed, and carbon is deposited on the catalyst. Hulea et al. performed the conversions of CH 3 SH to hydrocarbon (M2TH) and CH 3 OH to hydrocarbon (MTO) to compare the reactivity of CH 3 SH with that of the similarly structured CH 3 OH. 11 M2TH and MTO have many similarities in their selectivities toward aromatics and coke, but exhibit signicant differences in selectivities toward olens and paraffins. It has also been reported that CH 3 SH has a better methylation capacity than CH 3 OH, and that the toluene selectivity is higher for methylation to benzene using CH 3 SH. 12 Although there are many attractive aspects of the catalytic process when CH 3 SH is used as a reaction material, most of recent studies have focused on the conversion of CH 3 SH with ppm-order concentration. For the practical application of the conversion of CH 3 SH, it is necessary to convert CH 3 SH with volume order. However, such a catalytic process has not been explored so far.
In this study, MFI-type zeolite was used as a catalyst for the conversion of CH 3 SH. To convert CH 3 SH, we focused on the catalyst for the MTO system. Recently, in MTO systems, catalysts with Ga-introduced MFI zeolite frameworks were found to be effective, where H-ZSM-5 catalysts exhibited the highest methanol conversion but low selectivity for olens, while H-[Ga]-ZSM-5 catalysts promoted the formation of olens, C 5 + , and aromatic fractions. 13 The introduction of Ga into its framework increased the selectivity of olens. Specically, the Al component of the MFI-type aluminosilicate was replaced with the Ga component by hydrothermal synthesis method. We investigated the CH 3 SH conversion properties of zeolite catalysts with varying amounts of Ga component. The catalysts were characterized by physicochemical characterization via X-ray diffraction (XRD), N 2 adsorption measurements, scanning electron microscopy (SEM), and evaluation of acidic properties via the temperature-programmed desorption (TPD) of NH 3 and measurement of pyridine adsorption spectra using Fourier transform infrared spectroscopy (FT-IR).

Characterization of zeolite
The structure of the prepared catalyst was determined via XRD (Ultima IV, Rigaku Co. Ltd.) using a Cu Ka radiation source. The specic surface area, pore volume, and pore size distribution were determined by N 2 adsorption-desorption at −196°C (3-ex: Micromeritics Instrument Co. Ltd.). Scanning electron microscopy (SEM) was performed on an electron microscope (JSM-IT700HR/LA, JEOL Co. Ltd.) operated at 15.0 kV to identify the morphology of the as-prepared zeolite catalyst.

Evaluation of the catalytic performance of CH 3 SH conversion
The prepared zeolites' catalytic performances, including the activity and selectivity, were examined in a conventional xedbed reactor. Aer setting 0.4 g of catalyst at the center of the quartz tube, the catalyst was dried at 500°C in the He ow. The reaction was then performed at 400-550°C under atmospheric pressure. The reaction condition was as follows: the CH 3 SH feed rate was 0.25 mL min −1 (SATP); each reaction gas ow was He/ N 2 /CH 3 SH = 5.0/4.75/0.25 mL min −1 . The gaseous reactants and products in the effluent gases (CH 3 SH, CH 3 SCH 3 , CH 4 , C 2 H 4 , C 2 H 6 , and aromatics) were collected using a microsyringe and subsequently injected into a chromatograph equipped with thermal conductivity and ame ionization detectors (GC-8A; Shimadzu Inc., Japan) and a packed column (VZ-7, a length of 6 m, GL-Science). Conversion and yield as well as selectivity are calculated, basing on below equations.
Here, F product out represents the ow rate of the product in the effluent gas, n i represents the carbon number of the product, F CH 3 SH represents the amount feed rate of CH 3 SH.

Characterization of acid property
Acidic sites of the zeolites was characterized by temperatureprogrammed desorption of ammonia (NH 3 -TPD) using a BEL-CAT-A (Microtrac BEL) instrument. Aer the activation treatment at 500°C in N 2 ow for 2 h, ammonia gas (20% NH 3 /Ar at 30 mL min −1 ) was supplied for 60 min at 50°C. Aer purging with He, the temperature was increased to 500°C at a rate of 10°C min −1 in He ow, and the desorption of NH 3 was detected by a thermal conductive detector (TCD

DFT calculation
To estimate the nature of acidic sites in H-[Ga]-ZSM-5 zeolite, ab initio calculations were performed using the rst-principles calculation code "Quantum ESPRESSO." Projector-augmented wave (PAW) pseudopotentials were used to describe the core electrons. A plane-wave cutoff energy of 350 eV was selected, and a 289-atom unit cell for zeolite and 1 × 1 × 1 k-point mesh was used. The positions of the atoms and lattice parameters of each cell were optimized. size of about 1.5 mm. Such a spherical shape has been reported in Ga-substituted ZSM-5. 14,15 This morphology could be attributed to the deposition of small crystals from secondary nucleation on top of the initially formed larger units, retaining the contours of the original morphology. 16 When Si/Ga exceeded 200, the particle size gradually decreased to around 0.8 mm and the shape became a typical-hexagonal crystals of MFI-type zeolite. Fig. 3 shows N 2 adsorption-desorption isotherms for zeolite catalysts with various Ga contents. The BET surface area of each catalyst is shown in Fig. 3    characteristics of type I and IV isotherms. A steep uptake corresponds to micropore lling in the low-pressure region, followed by a hysteresis loop with increasing N 2 pressure. This indicates the presence of both micropores and mesopores in the H-[Ga]-ZSM-5 zeolite. The lesser the amount of Ga introduced, i.e., the higher the Si/Ga ratio, the more step-like N 2 adsorption is observed in the catalyst. ESI † shows the pore-size distribution of H-[Ga]-ZSM-5 catalysts. The catalyst with a higher Si/Ga ratio indicates narrow pore size distribution in the micro-pore region. An increase in the N 2 content above a relatively high pressure of approximately 0.91 was observed in all samples, indicating the possibility of the presence of macropores in H-[Ga]-ZSM-5 catalysts. 17 The acidic property of the H-[Ga]-ZSM-5 zeolite was evaluated to infer the change in the acidity and location of Ga in the zeolite structure. Specically, NH 3 was adsorbed on the zeolite catalyst at 50°C, following which the properties, such as strength and amount of acid sites, were measured from its desorption behavior. . The intensity of LT is considered to be proportional to the intensity of the HT in TPD spectra. 18 In the case of H-ZSM-5, the LT peak is thought to be caused by weak acidic silanol groups or by extra-framework aluminum oxide species. [19][20][21][22][23][24] Considering H-[Ga]-ZSM-5 with high Ga incorporation (= low Si/Ga), it has been stated that Ga can either exist as GaO, in an aggregated form on the external zeolite surface, as small particles within the zeolite pore, as an oxidizing agent GaO + , as a reducing Ga + species, or as GaH 2 . For example, the ion exchange, impregnation, physical admixture, and chemical vapor deposition of GaCl 3 oen produce several Ga species. [25][26][27][28][29] In ZSM-5 with higher Ga incorporation (= low Si/Ga), all Ga was not incorporated into the structure, and extra-framework Ga species and structural defects were produced, which might enable NH 3 desorption on the LT side. In contrast, in Ga-ZSM-5 (Si/Ga = 200), where the LT and HT areas are small, it is expected that most of the Ga is incorporated into the framework due to the small amount of Ga, resulting in a smaller LT area. In addition, the shi to LTs at high Si/Ga ratios is assumed to be due to the weakening of the acidity. Furthermore, based on the TPD prole, the number of acid sites in Ga-ZSM-5 (Si/Ga = 400, 600) was decreased.

Evaluation of physicochemical properties of Gaincorporated MFI zeolite
To further investigate the acidic properties of H-[Ga]-ZSM-5, FT-IR measurement was performed. Pyridine is used as a probe molecule for determining Brønsted acid (BA) sites and Lewis acid (LA) sites. The band at 1440-1470 cm −1 was reported to be assigned to adsorbed pyridine on LA sites, and the 1520-1560 cm −1 band was assigned to the protonated pyridinium ion on BA sites. 30,31 FT-IR peak at 1458 cm −1 was attributed to pyridine interacting with the LA site in H-[Al]-ZSM-5 and H-[Ga]-ZSM-5 with Si/Ga of 50. While for catalysts with Si/Ga greater than 100, only the BA sites were identied. The disappearance of the LA sites with higher Si/Ga ratios might indicate the successful incorporation of Ga into the MFI zeolite framework (Fig. 5).

Methanethiol conversion characteristics
To investigate the effect of Ga incorporation in the MFI zeolite framework for CH 3 SH conversion, catalytic activity tests were performed using H-[Ga]-ZSM-5 with different Si/Ga ratios. The C mol% yields of each product at a reaction temperature of  500°C are shown in Fig. 6. The high product yield was obtained for all catalysts, but the total yield was decreased as Si/Ga ratio was increased: 70.6% (Si/Ga: 50); 71.8% (Si/Ga: 100); 53.0% (Si/ Ga: 200); 36.1% (Si/Ga: 300); 22.7% (Si/Ga: 400); 18.2% (Si/Ga: 600). This decrease can be attributed to the decrease in the number of acid sites. The main products were DMS, methane, and other compounds, including unquantied trimethylbenzene, naphthalene, etc. (denoted as "Other"). DMS is a product obtained by the following reaction (eqn (4)), which is unsuitable for olen production.
CH 4 is assumed to have been generated during the decomposition of DMS. According to the report by Ohshima, DMS decomposition occurred catalytically on the acid site. 32 The proposed mechanism was as follows: CH 3 SH + (CH 3 ) + -a / H 2 S + CH 4 + C + H + -a The reaction (2) shows DMS decomposition to CH 3 SH over the Brønsted acid site, and reaction (3) shows CH 3 SH decomposition to H 2 S and CH 4 . These reactions proceed sequentially over the Brønsted acid site, and CH 3 SH formation shows the behavior of the primary product. For the H-[Ga]-ZSM-5 catalyst, the yields for other products (i.e., "other yields") were high for Si/Ga = 50 and Si/Ga = 100. However, the catalysts with Si/Ga ratio higher than 200 showed a signicant decrease in the other product's yield, an increase in the ethylene yield, and a decrease in the DMS yield. According to Baltrusaitis, it was estimated by DFT calculations that the main product, ethylene, is formed via the formation of trimethylsulfonium ion as the key reaction intermediate. 33 A relatively high ethylene yield was considered to be obtained by the formation of trimethylsulfonium ion, suppression of the reaction (eqn (4)) and sequential reaction to other compounds containing polycyclic aromatic molecules.   The yields of CH 4 and C 2 H 4 increased with increasing temperature. No other products were observed over Ga-ZSM-5 from 400 to 500°C. However, the formation of other substances was conrmed at temperatures $525°C. This is probably because of the polymerization of the produced ethylene to form carbon precursor species. The formation of C3 compounds was also observed at higher temperatures, which is also assumed to be due to the ETP reaction progressing over the zeolite acidic site. To consider the role of Ga in ZSM-5, we focused on the byproduct of DMS. It has been proposed that the production of DMS is related to the Brønsted acid property according to eqn (7) and (8). The reaction proceeds via a carbonium ion mechanism. 9 CH 3 SH + H + / CH 3 SH 2 + (7) CH 3 SH 2 + + CH 3 SH / H + + (CH 3 ) 2 S + H 2 S The protonation reaction (eqn (7)) is suppressed due to the low acid strength by substituting Al for Ga, which decreases the formation of DMS. Furthermore, the decrease in the number of strong acid sites due to coking during the decomposition of DMS to CH 4 on the zeolite might have reduced the amount of CH 4 production. [35][36][37] Additionally, the weakening of the property of the acidic sites by substituting Al for Ga suppresses the sequential reaction of ethylene to produce coke on the catalyst. The relatively good performance of H-[Ga]-ZSM-5 (Si/Ga = 200) is assumed to be due to its ability to suppress the formation of by-products and the sequential reaction of ethylene, thereby producing ethylene even with low acidity.

Conclusions
With the aim of industrial application of CH 3 SH, we investigated the reaction characteristics of gallium-substituted zeolite (H-[Ga]-ZSM-5) catalysts under conditions of high raw material (CH 3 SH) concentrations. DMS, aromatics, and CH 4 were formed as byproducts on the H-[Al]-ZSM-5 as the reference catalyst. The introduction of Ga into the ZSM-5 structure provided a high ethylene yield of 53.0% with relatively high selectivity of 36.2%. Based on the TPD of NH 3 and DFT calculations, the strength of acid sites was decreased by introducing Ga into the ZSM-5 structure. Undesirable reactions such as the formation of DMS and the sequential reaction of ethylene to produce aromatics seemed less likely to occur at weakly acidic sites, which provided a high ethylene yield on H-[Ga]-ZSM-5.

Conflicts of interest
There are no conicts to declare.